Hi,
I am sorry, I don't have any suggestions.
With my own father, I have often felt as though I was banging my head against a brick wall, but his last HbA1c was down a bit, which means he is doing something different - but I wouldn't claim any credit for that. He is the one who makes the decisions on what he eats, and so on.
What I suspect happened, was that last summer he felt very tired and was limited in what he could do in the garden - and he LOVES his garden - which may have given him the motivation he needed to reduce the raised blood glucose. Another motivation may have been him finding driving more challenging, and their car is their lifeline, because of where they live.
So how is that relevant to your father?
Only that until your father finds something that motivates him to change, he won't. No idea what that motivation may be, but it will take him wanting to change before he makes any changes.
Sorry I can't help more. I completely understand how distressing and frustrating it is, to watch the situation unfolding.